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Appendix I.


of expansion and compression between these temperatures are
•strict adiabatics.

The second and only* other method of obtaining reversibility
is to use a regenerator, as first practically attempted by Stirling, and
•afterwards by Ericsson in their respective air engines. Diagram
-A, Fig. 757, shews Stirling's cycle. The gas having a pressure



and volume corresponding to #, took in heat along the isothermal
-a b; rejected a portion during b c to the regenerator, at constant
volume; was compressed isothermally from c to d, during which
time heat was rejected to a refrigerator; and between d and a
again received the heat which was rejected from b to c. Although
«J> c and d a are substituted for adiabatics, the giving and receiving
of heat is strictly within the engine itself, the heat rejected, at b c
-being fully returned at d a, so the reversibility is unimpaired.
Diagram B illustrates the cycle of Ericsson, which is only
-different from that of Stirling in that the regenerator gives or
abstracts heat at constant pressure. As before, a may be con-
sidered the starting-point. * (Excepting as in Apfendix If.,
A 883.)

P. 613. Reversibility in the Steam Engine. — It
appears, therefore, that supply and rejection must be along
isothermals, and expansion and compression along adiabatics
(unless a regenerator be used). Remembering that isothermals
fbr saturated steam are horizontal straight lines, the reversible
cycle in Fig. 758 for the steam engine is easily understood.
Thus a b is the isothermal of reception, b c the adiabatic of
expansion, cdiht isothermal of rejection, and da the adiabatic